Colorless, transparent objects (phase objects) such as cells and glass bodies, which are hard to observe according to an intensity distribution (amplitude image) of transmitted light occurring when the objects are irradiated with light, are observed according to a phase distribution (phase image) of the transmitted light. Examples of devices for observing the phase image of such an object include phase contrast microscopes and differential interference microscopes. However, these devices cannot obtain quantitative information about the optical thickness of the objects.
As techniques for obtaining quantitative information by observing a phase image of an object, those using phase shift methods described in Patent Literature 1 and Non Patent Literature 1 have been known. In observation devices based on these phase shift methods, light having a wavelength λ emitted from a light source is split in two, one split light is transmitted through the object so as to become object light, the other split light is used as reference light, and a two-dimensional image formed by interference between the object light and reference light is captured. While changing the optical path length of the reference light in increments of λ/4, four two-dimensional images are obtained, which are then subjected to a predetermined arithmetic operation, so as to yield amplitude and phase images of the object.